The field emission device consists of a cathode or emitter electrode, a gate or accelerating electrode and an anode or collector electrode. In general, the distance between the emitter and the accelerating electrodes is extremely small--in the order of one micron or less. As a result, a small voltage applied between the two electrodes generates a strong electric field. Electrons are emitted from the emitter electrode under the action of a strong electric field, a process called field emission. The difference between field emission and thermionic emission is that in thermionic emission, electrons are emitted from a hot electrode as a result of the heat. Hence the kinetic energy of the electrons is higher than that of the potential barrier between the electrode material and the vacuum. In field emission, electron emission is caused by a strong electric field so that a hot emitter electrode is not required. Thus the major advantage of field emission over thermionic emission is that a small electron source unit without the heating element can be constructed.
There are several prior art methods which can be used to fabricate silicon field emission devices. One method is to start with a silicon substrate, 1, on which silicon dioxide islands, 2, with a diameter of say 1 micron are patterned (FIG. 1). A silicon etching process is carried out until the diameter of silicon under each oxide island, 3, is in the order of 0.2 micron, with the oxide islands being used as an etching mask (FIG. 2). A dielectric layer, 4, of 0.3 micron thickness, for example, silicon dioxide or silicon nitride, is deposited by thermal evaporation or chemical vapour deposition on the entire substrate (FIG. 3). This layer will cover the top of the oxide island and around the oxide islands but will not trickle beneath the oxide islands. A second silicon etching process is carried out and a tip emitter electrode, 5, is formed. A layer of metal, 6, such as aluminum or chromium, is then deposited on the entire surface (FIG. 4). As a result, the metal sitting on the dielectric layer acts as the gate or accelerating electrode.
This prior art method does not require a photolithographic alignment process, however, the breakdown voltage of the dielectric layer deposited by thermal evaporation or chemical vapour deposition is not as high as that of thermally grown silicon dioxide. Therefore, a field emission array fabricated using the above method is not efficient enough for long term device operation. If an electrical breakdown between the gate and the substrate occurs, the corresponding field emission device becomes defective. It is thus highly desirable to develop a process which utilizes thermally grown oxide as the insulator between the gate and substrate.